Although the stator windings of electrical inductive devices, such as motors, are wound with magnet wire having enamel or other insulative coating thereon, it is often desirable to further coat the windings and seal them from the environment. For example, protection of the stator windings by an additional coating is desirable in blower motors utilized in cooling systems for locomotive traction motors. Protection is also desirable in open motors utilized in driving pumps in oil field applications, which are exposed directly to blowing sand and dirt as well as moisture.
Conventional curable varnish compositions, such as those used in certain locomotive traction motors, are so-called “solventless” varnishes based on unsaturated polyester resin (UPR). These varnish systems, however, have a glass transition temperature (Tg) significantly below 80° C. Thus, their performance at motor operating temperatures, usually about 160° C., can result in significant thermal degradation after extended operating times. In addition, such varnishes tend to chip or crack, particularly when subjected to vibrations accompanying locomotive operation. Such UPR varnishes also have a high moisture absorption rate and their ester bonds are hydrolysable, which can result in more frequent maintenance intervals than desired.
What is needed is a varnish composition that can better withstand higher temperature and a method for electrically insulating electrical devices with the varnish composition. Polyphenylene ether resins (hereinafter “PPE”) are commercially attractive materials because of their unique combination of physical, chemical, and electrical properties. Furthermore, the combination of PPE with other resins can provide blends that result in additional overall properties such as chemical resistance, high strength, and high flow. However, the use of PPE, especially of low intrinsic viscosity, is relatively expensive for use in a varnish composition.
One method of making a PPE of low intrinsic viscosity is by the redistribution of the PPE by reaction with a phenolic compound. For example, UK 1 119 914 describes the redistribution of poly(phenylene ether) with phenolic compounds, designating the redistribution by the term equilibration. In the redistribution reaction, poly(phenylene ether) is dissolved in a solvent with a phenolic compound and a promoter and heated at elevated temperatures. The poly(phenylene ether) polymer is split into shorter units, wherein the phenolic compound is incorporated into the poly(phenylene ether).
More recently, redistribution of poly(phenylene ethers) employing certain classes of phenolic compounds to obtain oligomers, using toluene or a blend of toluene and an alcohol as solvent, was disclosed in U.S. Pat. No. 5,880,221. After redistribution, the poly(phenylene ether) was isolated by precipitation or evaporation of a solvent mixture.
It would be desirable to develop a commercially attractive process for preparing a varnish composition comprising PPE. It should be apparent that each process step and associated mechanical equipment such as reaction vessels used in preparing the PPE and the varnish composition are a significant consideration in determining the commercial feasibility of using PPE in a varnish composition.